The Syrian military’s movement of chemical weapons in recent days has prompted the United States and several allies to repeat their warning to President Bashar al-Assad that he would be “held accountable” if his forces used the weapons against the rebels fighting his government.

The warnings, which one European official said were “deliberately vague to keep Assad guessing,” were conveyed through Russia and other intermediaries....

One American official provided the most specific description yet of what has been detected, saying that “the activity we are seeing suggests some potential chemical weapon preparation,” which goes beyond the mere movement of stockpiles among Syria’s several dozen known sites. But the official declined to offer more specifics of what those preparations entailed.

Over the weekend, the activity in Syria prompted a series of emergency communications among the Western allies, who have long been developing contingency plans in case they decided to intervene in an effort to neutralize the chemical weapons, a task that the Pentagon estimates would require upward of 75,000 troops. But there were no signs that preparations for any such effort were about to begin.